Magnetic anomalies emanating from the Bermuda Triangle.
Anti-gravitational forces caused by UFOs. An "ineffable,
natural phenomenon that cannot be described or explained."

Supernatural gabble gushes from the house of illusions in
Santa Cruz known as the "Mystery Spot," one of more than a
dozen places in the nation where stunning visual illusions
compel people to reach for metaphysical explanations to
describe their experiences.

At Santa Cruz's "Mystery Spot," balls roll uphill, chairs
sit on walls and people lean over so far they can't see
their shoes, yet they don't fall down.

Nineteenth-century psychologists had theories to explain
illusions like this, but the explanations left considerable
room for mystery.

Now, Berkeley psychologists have generated a new theory
based on experimental data that goes much further in
explaining all the effects of the phenomena known
collectively as "the mystery spot."

Central to their thesis is a new emphasis on the human
need to establish horizontal and vertical orientations and
the extent to which people take their cues from the
immediate context if they can't see the earth's horizon.

"All the visual illusions in the Mystery House derive
from the fact that the house is tilted," said William
Prinzmetal, adjunct associate professor of psychology. He
conducted the studies with colleague Arthur Shimamura, also
a psychology professor.

"You know the house is tilted, but you don't know how
much. Everything is tilted. You can't look outside and get a
horizon, so you think that what you see is right. It's very
compelling," said Prinzmetal, an expert on perception who
has been to the Mystery Spot a dozen times. Although he has
studied these illusions, he said his visual perceptions
still are distorted when he goes into the house, which is
tilted at a 20-degree angle from the ground.

It doesn't take a scientist to know that cockeyed rooms
affect perception. If floors are slanted, for instance,
people will hang pictures on a slant.

But what has not been known before is that when the
perceiver's body also is tilted, the distorting impact on
vision is greatly magnified -- up to two or three times the
effect of slanting the visual field alone.

"In the tilted condition, you are much more affected by
the immediate visual context," said Prinzmetal, who has
tested dozens of subjects in a laboratory chair tilted at a
30-degree angle. In that position, he tests their ability to
line up vertical dots in a slanted matrix in a darkened room
where they have no clue to the true horizon. With their
bodies tilted, he said, people's perceptual distortion more
than doubles, compared to when they see the same matrix from
a level chair.

"We are such visual animals," said Prinzmetal. "The
mechanism in us that's responsible for determining the
horizontal and vertical is mostly affected by what we see.
If the context is screwy, that will throw off what we see as
vertical and horizontal."

He said that other cues to people's horizontal
orientation, such as the vestibular system in the inner ear
and bodily sensations of gravity, appear to become less
functional in the tilted condition, leaving visual context
as the dominant cue.

Prinzmetal contends that understanding the principles of
the Mystery Spot is critical for understanding other visual
illusions that have remained unexplained for more than a
century.

These illusions can make lines appear longer or shorter
than they are, or straight lines appear curved and curved
lines appear straight, among other distortions of reality.

Many of these visual illusions are also increased by
sitting in the tilted chair, said Prinzmetal.

One critical application of the new research is to
improve the flying of airplanes.

All cockpits carry an "artificial horizon," essentially a
leveler, that pilots use when the real horizon is not
visible. It is the only clue to the horizon when the cockpit
is tilted -- as the chair was tilted in the laboratory.
Pilots are trained to ignore the visual context of the
cockpit and fasten their eyes on the leveler.

Unfortunately, they don't always follow that rule, said
Prinzmetal, adding that planes have crashed because the
pilot thought he was flying level when actually he was at an
angle.

That is said to have happened in one story circulating
through government aeronautic circles. According to the
tale, the last words heard on the tape retrieved from a
crashed airliner was the navigator shouting, "Look at your
artificial horizon!" and the pilot responding, "I can't.
It's broken!"

Prinzmetal said that scientists at the National
Aeronautics and Space Administration, with new appreciation
for the strength of these visual illusions, now are working
to make the displays for artificial horizons much more
obvious.